Injection-expansion molded shotshell wad and method of forming the same

ABSTRACT

A one-piece plastic wad structure for use with a shotshell, the wad structure having a projectile-receiving pocket, an obturating pocket, and a transverse wall integral with the pockets and separating one pocket from the other. The projectile pocket and obturating pocket have side walls of solid plastic, and at least a portion of the transverse wall is foamed plastic. The transverse wall is of substantial thickness so as to cushion the shot charge when a cartridge containing the wad is fired. The wad structure is formed by injecting a molten mixture of a resinous material and a foaming agent into a mold cavity, cooling the injected mixture so as to partially solidify the mixture in the cavity, and then reducing pressure in the cavity by enlarging its volume to permit the transverse wall portion of the injected mixture to foam in situ.

United States Patent 91 Hughes et al.

[ ]March 20, 1973 INJECTION-EXPANSION MOLDED SHOTSHELL WAD AND METHOD OF FORMING THE SAME [75] Inventors: John H. Hughes, St. Louis, Mo.;

Daniel F. Duryea, Madison, 11].

[73] Assignee: Olin Corporation [22] Filed: Dec. 4, 1970 [21] Appl. No.: 95,073

1,262,020 4/ 1960 France 102/95 1,408,820 7/1965 France 1,109,193 4/1968 Great Britain ..102/42 C [57] ABSTRACT A one-piece plastic wad structure for use with a shotshell, the wad structure having a projectile-receiving pocket, an obtaratingpocket, and a transverse wall integral with the pockets and separating one pocket from the other. The projectile pocket and obturating pocket have side walls of solid plastic, and at least a portion of the transverse wall is foamed plastic. The transverse wall is of substantial thickness so as to cushion the shot charge when a cartridge containing the wad is fired. The wad structure is formed by injecting a molten mixture of a resinous material and a foaming agent into a mold cavity, cooling the injected mixture so as to partially solidify the mixture in the cavity, and then reducing pressure in the cavity by enlarging its volume to permit the transverse wall portion of the injected mixture to foam in situ.

10 Claims, 8 Drawing Figures PATENIEDHARO 1915 3. 721, 197

SHEET 1 or 3 III I 14 3 FIG [I6 4 FIG I FIG 2 a) INVENTORS JOHN H. HUGHES DA ME]. I? DURYEA ATTORNEY PATENTED MR 2 0 I975 3.721.197 SHEET an; 3

FIG '6 INVENTORS JOHN H. HUGHES DANIEL FDURYEA BY wm- 121M ATTORNEY m A/MM mwm w m w w n M M w W %Aj\\ MK a 3 3% INJECTION-EXPANSION MOLDED SI-IOTSHELL WAD AND METHOD OF FORMING THE SAME This invention relates generally to wad structures for use with shotshells, and more particularly to a unitary plastic wad structure having a projectile-receiving cup portion of solid plastic, a gas obturating cup portion of solid plastic, and a transverse wall integral with and separating the two cup portions, the transverse wall serving a cushioning function and being at least in part formed of foamed plastic.

The use of plastic wads as components of shotshells is generally old in the art. Such wads have been proposed in one-piece design having a solid plastic pro- "'je'ctile pocket or cup'for containinga shot pellet" charge, and an integral obturating cup of solid plastic for positioning over the powder charge in the shell to prevent blow-by of combustion gases when the shell is fired. The onepiece wads of the prior art also may in clude a transverse wall of solid plastic closing the bottom of the projectile cup and a transverse wall of solid plastic closing the top of the obturating cup. The onepiece wads having a pair of transverse walls also may include some form of cushioning means extending between the two cupsand integral with the two cups and formed from solid plastic. Some examples of this type of one-piece wad structure are shown in U.S. Pat. No. 3,368,489 to G. L. Herter, issued Feb. 13, 1968; U.S. Pat. No. 3,298,313 to R. W. Comerford, issued Jan. 17, 1968; and U.S. Pat. No. 3,285,174 to V. C. Moehlman et al., issued Nov. 15, 1966. Other types of one-piece plastic wads have been utilized wherein a common transverse wall of solid plastic closes off both the projectile cup and obturating cup and separates one from the other. For cushioning the latter type wad structure, a separate disc wad of foamed plastic, cork, fiber, or the like may be inserted into the projectile cup to underlie the projectile charge. An example of the latter type wad is,-shown in U.S. Pat. No. 3,309,994 to F.A. Lage, issued Mar. 21, 1967.

Composite, or multiple piece wad combinations of plastic have also been proposed by the prior art. Such multiple piece wad combinations conventionally may include a separate projectile cup having a bottom wall and formed entirely of solid plastic, a cushioning wad disposed below or in the projectile cup and formed from foamed plastic, cork, fiber, or the like and a separate over powder obturating cup formed from solid plastic and positioned between the cushioning wad and the powder charge in the shotshell. A structure of this general type is shown in U.S. Pat. No. 3,180,265 to R. A. Rybak, issued Apr. 27, 1965. i

The one-piece and composite wad structures referred to above have displayed various undesirable characteristics. In particular, the composite wad structures are time consuming and difficult to assemble and load. The performance of the unitary structures having solid plastic cushioning or shock-absorbing portions has been poor in that these structures have not displayed the required ability to function over the wide temperature range of -40 to 125 F., which temperature range represents the range of environmental temperatures at which the wads could be used.

The wad structure of this invention eliminates the undesirable traits of the prior art structures by providing a unitary wad structure which has an integral foamed plastic cushioning portion and which completely eliminates the assembly operations of the wad components required by the prior art composite wad structures. The wad structure of this invention has also exhibited satisfactory performance when fired at temperatures in the range of 40 to F. The wad includes a plastic over-powder gas obturating cup portion having its upper or inner end closed by a transverse wall which includes an integral foamed plastic portion. The longitudinal thickness of the foamed plastic portion can be varied to increase the amount of shock absorption provided when the wad is fired. The wad also includes a shot cup or pocket portion of solid plastic for receiving a charge of shot pellets. The transverse wall forms the bottom of the shot cup, and the side walls of the shot cup can be split to form petals. Both the upper and lower transverse peripheral portions of the transverse wall are preferably provided with a skin of solid plastic to toughen the transverse wall and to prevent shot pellets from becoming embedded in the foam when the wad is fired. In its most preferred form, the wad is a unitary body, however, it can be made from a plurality of components, for example, a gas obturating cup and a separate and distinct shot-receiving cup. In the latter instance one or both of the cups are formed with a transverse integral wall having a foamed plastic portion.

It is, therefore, an object of this invention to provide a plastic wad structure for use with an ammunition cartridge, which wad structure provides ease of assembly, superior performance over a wide temperature range, and improved projectile patterns.

It is another object of this invention to provide a wad structure of the character described which includes a gas obturating cup, a shot-receiving cup, and an integral transverse wall separating the cups and having an integral foamed plastic portion.

It is a further object of this invention to provide a wad structure of the character described wherein the foamed transverse wall provides a cushioning function and includes peripheral portions of solid plastic to form a tough skin thereon.

These and other objects and advantages will be readily apparant from the following detailed descrip tion of several preferred embodiments of the invention along with the accompanying drawings, in which:

FIG. 1 is a vertical sectional view of a preferred embodiment of the wad structure of this invention which is unitary or one piece;

FIG. 2 is a vertical sectional view of the wad structure of FIG. 1 shown loaded into an assembled shotgun cartridge;

FIG. 3 is a vertical sectional view of another embodiment of the structure of this invention wherein the wad includes two distinct members, a shot cup and a gas obturating cup with the latter having the integral foamed plastic portion;

FIG. 4 is a vertical sectional view of yet another embodiment of the wad structure of this invention which is similar to the wad structure of FIG. 3, but wherein the foamed plastic portion is integral with the shot cup;

FIG. 5 is a vertical sectional view of a generally sche matic representation of the mold assembly used in making the wad structure of FIG. 1;

FIG. 6 is a vertical sectional view of the mold assembly of FIG. showing the positioning of the components thereof permitting foaming of the transverse wall of the wad structure;

FIG. 7 is a vertical sectional view of the mold assembly of FIGS. 5 and 6, but showing the manner in which the molded wad is removed from the mold; and

FIG. 8 is a vertical sectional view of a modified mold assembly wherein the components of the mold are arranged to permit foaming to occur toward the shot cup portion of the wad.

Referring now to FIGS. 1 and 2, a preferred onepiece embodiment to the wad structure of this invention is shown. The wad structure 2 includes a first pocket or cup portion 4 for tlie rec eption of a charge of projectiles. The side wall of the projectile cup maybe longitudinally sloted as at 6 to subdivide the wall into a plurality. of petals 8. The wad 2 also includes a second pocket or cup portion 10 having a side wall 12 in the form of a flexible plastic skirt which can flare outwardly against the cartridge casing and the barrel bore of the gun when the cartridge is fired, thus obturating the combustion gases propelling the wad. A transverse wall 14 extends across the wad 2 so as to form a bottom wall 16 for the projectile cup 4, and a top wall 18 for the obturating cup 10. The central portion or core 20 of the transverse wall 14 is formed of plastic foam, with the peripheral parts of the transverse wall 14 being formed of solid plastic, as at 22, 24 and 26. Thus the bottom wall .16 of the projectile cup 4 includes a tough solid plastic skin 24 into which projectiles will not embed when the wad is fired from a gun. Furthermore, the bottom wall 18 of the obturating cup 10 also includes a tough skin 26 of solid plastic which strengthens the cup 10 and resists splitting of the transverse wall 14 when the wad is fired. As shown in FIGS. 1 and 2, the foamed portion 20 of the transverse wall is sufficiently thick, or has sufficient longitudinal dimension so as to function as a filler wad, cushioning wad, and an aid to the obturation of combustion gases. The term solid plastic as used in connection with this invention refers to a mass of plastic which is substantially devoid of gas-filled voids or cells.

FIG. 2 shows a shot shell 28 into which the structure 2 has been loaded. The shot shell 28 includes a shell tube 30 of paper, plastic, or the like, onto which may be fitted a metallic head member 32. A base wad 34 of plastic, paper, or other fibrous material is positioned in the tube, and a primer cup 36 extends through the head 32 and base wad 34. The primer cup 36 opens into a charge of propellant 38, over which is positioned the obturating cup portion 10 of the wad 2. A charge of shot pellets 40 are positioned in the shot cup portion 4 of the wad 2, and a thin card 43 closes the top of the shell 28. Alternatively, the top of the tube 30 may be crimped orfolded inwardly to close the top of the shell 28, or any conventional top closure may be used with the wad of this invention. While the shell 28 shown is of the impact ignition variety, the wad structure of this invention can be used with other known types of shells, as for example, electrically ignited shells.

The shell 28 is loaded in a conventional shotgun and is fired by striking the primer cup 36 to ignite the primer therein and the propellant charge 38. Combustion gases formed by ignition of the propellant charge act upon the obturating cup 10 and force the skirt l2 outwardly into sealing engagement with the inside of the tube 30. The shot charge 40 is cushioned from the sudden shock of expanding combustion gases by the foamed part 20 of the transverse wall 14 of the wad, so that the wad 2 and shot charge 20 are smoothly expelled from the casing 30 and into and through the barrel bore without a sudden jolt which disrupts the shot charge 40. The shot cup 4 protects both the shot 40 and barrel bore from contact with each other, and falls to the ground shortly after leaving the muzzle of the gun in a known manner, with the shot charge continuing on in a normal trajectory.

FIG. 3 shows an alternative embodiment of the wad of this invention which includes two separate members which are positioned back-to-back when loaded into a cartridge. The first member is a substantially conventional shot cup member 42 which may be formed from paper, plastic or some other suitable material, the cup 42 may include a bottom wall 44 or may be devoid thereof. The second member is a plastic obturating cup 46 which has a transverse wall 48 with an integral foamed plastic portion 50. The obturating cup 46 has a thin flexible skirt 52 which operates in the manner previously described, and there is a solid plastic skin 54 and 56 on either face of the foamed part 50 of the I transverse wall 48. The transverse wall 48 is of substantial thickness so as to provide cushioning when the wad is fired.

FIG. 4 shows yet another alternative embodiment of the wad structure of this invention which also includes two separate members arranged back-to-back when loaded in a shell. The first member is an obturating cup 58 having a transverse wall 60. The obturating cup 58 may be made of plastic, paper, or other suitable material and may be of conventional construction. The second member is a projectile-receiving cup 62 of plastic and having a transverse wall 64 of substantial thickness for cushioning with an integral foamed plastic portion 66. There is a solid plastic skin 68 and 70 formed on either transverse face of the foamed part 66 of the transverse wall 64. i

The wad structures shown in FIGS. 3 and 4 gives excellent cushioning to the shot charge when fired. It is noted that the separate components of the embodiments shown in FIGS. 3 and 4 can be joined together with adhesive or the like, or can be simply positioned in a shell tube in back-to-back arrangement.

Referring now to FIGS. 58, there is shown a schematic representation of a mold assembly adapted to make the wad structures shown in FIGS. 1-4, and illustrating the various steps in the preferred method of manufacture thereof. The mold assembly includes a plurality of die members 72, 74 and 76. A passage 78 extends along a common axis through the die members 72, 74 and 76. A mandrel 80 is secured to the die member 72 and extends into the passage 78, the mandrel 80 having a portion 82 of reduced diameter which combines with the wall of the passage 78 to form a part of a mold cavity 84. A sprue 86 extends through the die member 72 and branches laterally as at 88 to open into the mold cavity 84 through a gate 90. It is noted that a number of branches may extend like spokes from the sprue 86 to a plurality of separate mold cavities to concurrently permit formation of a number of wads. A plug 92 having an axial bore 94 extends into the passage 78 through the die members 74 and 76. The inner face of the plug 92 is frustoconically shaped as at 95 and forms a part of one end wall of the mold cavity 84. A plunger member 96 is positioned in the plug bore 94 with the inner end wall of the plunger blending smoothly with the plug face 95 to complete one end wall of the mold cavity 84. Coil spring means 100 is mounted about the plunger 96 to bias the latter toward the right, as viewed in FIGS. 5-7. It is apparent from FIG. 5 that the passage 78 and mandrel 80 combine to form the projectile cup portion of the cavity 84, the mandrel 80 and plunger 96 combine to form the transverse wall portion of the cavity 84, and the adjacent end walls of the plug 92 and plunger 96 combine with the wall of the passage 78 to form the obturator cup portion of the mold cavity 84. A tubular member 102 having a central passage (not shown) is secured to the die member 72, with the central passage of the member opening into the sprue 86 to provide means for introducing a mixture of melted plastic resin and foaming agent into the cavity 84. The member 102 may also be connected to a drive mechanism (not shown) such as a hydraulic cylinder, or the like, to cause movement of the die member 72, as will be more clearly explained hereinafter.

A clevis or fork 104 is formed on the upper surface of each of the die members 74 and 76, and a pair of actuating rods 106 are pivotally connected to each of the forks 104' by means of pins 108. An actuating piston 110 is reciprocally mounted in an actuating cylinder 112 and pivotally connected to each of the rods 106 by means of a pin 114. A timer assembly 116 of any conventional construction is connected to the cylinder 1 12 by means of conductors 1 18.

The wad structure is formed in the following manner. A mixture of suitable melted resinous plastic material and a foaming agent is injected under pressure through the sprue 86, branch 88 and gate 90 into the mold cavity 84 to completely fill the latter. The plastic material used is pre-mixed with the foaming agent. The injected plastic and foaming agent mixture is confined in the mold cavity 84 under super atmospheric pressure and cooled sufficiently to cause formation of a skin of solid plastic throughout the entire periphery of the confined charge. The duration of the time period of such confinement is governed by the timer 116. The injected charge can be cooled by any conventional means, such as cooling coils (not shown), or the like. At the end of the predetermined time period, the timer 116 causes actuation of the actuating cylinder 112 to push the actuating piston 110 downwardly, as shown in FIG. 6. Downward movement of the piston 110 in turn causes the rods 106 to move apart thus forcing the die member 76, plug 92, and plunger 96 to move away from the die members 72 and 74. The die member 76 is preferably slidably mounted 'on suitable guide rails or guide rods (not shown) so that the plunger 96 and plug 92 remain axially aligned with the mandrel 30. The movement of the respective end faces 95 and 98 of the plug 92 and plunger 96 away from the opposite end face of the mandrel 80 results in enlargement of the volume of the mold cavity 84 in the vicinity of the. transverse wall portion thereof and a concurrent reduction in the mold cavity pressure. The thusly enlarged volume and reduced pressure then permits the plastic material in this vicinity to foam in situ. Thus, prior to complete solidification of the molten plastic in the mold, the movable core section is retracted allowing a pressure relief to occur in the confined cavity. After the pressure has been released by moving the cored section, the heat and pressure sensitive foaming agent liberates a gaseous mixture causing the molten portion of the plastic to foam producing a cellular air-foam matrix with a density of about one-third the density of the base resin. The mold cavity 84 is then retained at its enlarged volume and further cooled until the solid plastic and foamed plastic confined therein have substantially completely solidified, after which time the member 102 is caused to move away from the die member 14, thus pulling the die member 72 and mandrel with it, as shown in FIG. 7. In this manner the solidified plastic wad structure and attached plastic sprue are withdrawn from the projectile cup portion of the mold cavity. The plunger 96 is then moved against the bias of the spring 100 and against the solidified wad structure to push the latter free from the remainder of the mold cavity. The plastic sprue is then trimmed from the wad structure to form the finished product.

It has been noted that a thicker solid plastic skin will form adjacent to the wall of the mold cavity which remains stationary, and a thinner skin will form adjacent to the wall of the mold cavity which is moved. Thus, if one were desirous of forming a wad structure having a thicker solid plastic skin on the obturator cup, the molding arrangement shown in FIG. 8 would accomplish this result. In this molding arrangement, the inner face of the mandrel 80 forms the obturating cup portion of the mold cavity 84, and the plunger 96 and plug 92 combine with the passage 78 to form the projectile cup portion of the mold cavity. The manner in which the molding assembly shown in FIG. 8 is utilized is substantially identical to the molding procedures described above.

The wad structure of this invention can be made from such plastics as ethylene-vinyl acetate copolymer, copolymers of ethyl acetate, polyethylene copolymerized with butyl rubber, crosslinked polyethylenes, and equivalent resinous materials. The foaming agents which are mixed with the plastic prior to injection may include axodicarbonamide, P, P'- Oxbis (benzene sulfonyl hydrazide), azobis (isobutyronitrile), N,N'-dinitrosopentamethylenetetramine, and the like. The resin and foaming agent are blended together prior to being injected into the mold cavity in a standard reciprocating screw molding machine. The density of the foam portion of the preferred wad structure of this invention falls into the range of about l0 lbs/ft. to about 35 lbs/ft, and it is noted that the foamed portion of the wad structure is a closed cell foam. Since the density of the foamed portion of the wad is less than the density of the solid plastic parts thereof, the resulting structure is of substantially lighter weight than if made completely from solid plastic, or with solid plastic or fibrous filler or cushioning wads. While being of lighter weight, the wad structure also exhibits superior strength and buffering or shock-ab sorbing characteristics.

A specific wad structure of the type shown in FIG. 1 having solid plastic parts with a density of about 57 lbs./ft and having a cross-sectional bulk density in the foam cushion portion of about 15 lb./ft was made in the following manner. A pelletized charge of a low density polyethylene-vinyl acetate copolymer was mixed with a charge of powdered azodicarbonamide foaming agent by tumbling the two together. The mixture was 99 percent by weight of the copolymer and 1 percent by weight of the foaming agent. The mixture was then fed into a screw-type molding machine where it was heated sufficiently to convert the mixture into a viscous melt, the temperature of which is above the decomposition temperature of the foaming agent. The molten mixture was then injected into the mold cavity and maintained therein at a pressure of about 5,000 psi while the mold cavity and enclosed charge were cooled sufficiently to solidify the thin-walled areas of the confined charge, and to form a solid plastic skin at the periphery of the confined charge. The volume of the cavity was then enlarged sufficiently to lower the pressure within the cavity to less than about 500 psi. The decomposed foaming agent in the still molten area of the charge then liberated a gas which caused foaming of the molten portion of the encapsulated charge. The solid plastic-foam composite was then retained in the mold cavity and cooled further until the entire composite had substantially solidified. The solidified composite was then removed from the mold cavity and trimmed to form the final product. Experimentation has shown that superior cushioning and weight are achieved when the cross-sectional bulk density of the foamed cushioning portion of the wad is within the range of about 20 percent to about 60 percent, of the density of the solid plastic portion of the wad.

The wad structure 'of this invention thus exhibits several advantages over the prior art wad structures, in particular, light weight, ease of loading, superior cushioning, and absence of linear gaps in loaded wad components, to name a few. The wad structure can be easily mass produced at low cost, and automatically loaded into a cartridge. Satisfactory gas sealing, shot cushioning, and shot protection during passage through the gun barrel are also provided by the wad structure of this invention.

Since many changes and variations of the disclosed embodiments of the invention may be made without departing from the inventive concept, it is not intended to limit the invention otherwise than as required by the appended claims.

What is claimed is:

1. A plastic wad structure for use in a cartridge, said wad structure including: a one-piece member comprising a tubular side wall of substantially solid plastic; and a transverse wall spanning said side wall to form a cupshaped member, said transverse wall including a skin portion of substantially solid plastic unitary and in one piece with and of the same composition as said side wall, said skin forming an inner transverse surface of said cup-shaped member, and said transverse wall further including a plastic cellular foam portion unitary and in one piece with and of the same composition as said skin portion.

2. The wad structure of claim 1, wherein said plastic skin portion substantially completely encapsulates said plastic foam portion of said transverse wall.

3. The wad structure of claim 1, wherein said plastic foam portion has a density in the range of about 10 lbJfr" to about 35 lb./ft.

. The wad structure of claim 1, wherein sald cupjected to high pressure gases upon being fired from a cartridge for obturating such high pressure gases, said transverse wall including a plastic cellular foam portion and a skin portion of substantially solid plastic unitary and in one piece with and of the same composition as said foam portion, said skin portion forming at least an inner surface of said cup-shaped member which inner surface is impinged by the high pressure gases upon firing.

6. The wad structure of claim 5, wherein said skin portion substantially completely encapsulates said foam portion.

7. The wad structure of claim 5, wherein said plastic foam portion has a density in the range of about 10 lb./ft to about 35 lb./ft.

8. A one piece plastic wad structure for use in a cartridge, said wad structure comprising: a tubular side wall of substantially solid plastic; and a transverse wall spanning said side wall to form a cup-shaped member, said transverse wall including a core of foam plastic unitary and in one piece with and of the same composition as said side wall, and a skin portion of substantially solid plastic unitary and in one piece with and of the same composition as said side wall and substantially completely encapsulating said foam portion of said transverse wall.

9. A unitary wad structure for use in a cartridge, said wad structure comprising: an annular side wall of substantially solid plastic; and a transverse wall unitary and in one piece with said side wall and dividing the latter into back-to-back cups, said transverse wall including an inner core of foam plastic of the same composition as said side wall and an outer skin portionof substantially solid plastic of the same composition as said side wall, said skin portion substantially completely encapsulating said foam plastic core.

10. A plastic wad structure for use in a cartridge, said wad structure comprising a one piece member having a projectile-receiving cup portion of substantially solid plastic; a gas obturating cup portion of substantially solid plastic; and a transverse wall unitary and in one piece with and separating said cup portions, said transverse wall being at least in part formed of foam plastic unitary and in one piece with and of the same composition as said solid plastic of said cups. 

1. A plastic wad structure for use in a cartridge, said wad structure including: a one-piece member comprising a tubular side wall of substantially solid plastic; and a transverse wall spanning said side wall to form a cup-shaped member, said transverse wall including a skin portion of substantially solid plastic unitary and in one piece with and of the same composition as said side wall, said skin forming an inner transverse surface of said cup-shaped member, and said transverse wall further including a plastic cellular foam portion unitary and in one piece with and of the same composition as said skin portion.
 2. The wad structure of claim 1, wherein said plastic skin portion substantially completely encapsulates said plastic foam portion of said transverse wall.
 3. The wad structure of claim 1, wherein said plastic foam portion has a density in the range of about 10 lb./ft3 to about 35 lb./ft.3
 4. The wad structure of claim 1, wherein said cup-shaped member is a projectile-receiving cup and said tubular side wall is slotted to form petals.
 5. A wad structure for use in a cartridge, said wad structure comprising: a one-piece cup-shaped member having a transverse wall unitary and in one piece with a cylindrical skirt, operative to flare outwardly when subjected to high pressure gases upon being fired from a cartridge for obturating such high pressure gases, said transverse wall including a plastic cellular foam portion and a skin portion of substantially solid plastic unitary and in one piece with and of the same composition as said foam portion, said skin portion forming at least an inner surface of said cup-shaped member which inner surface is impinged by the high pressure gases upon firing.
 6. The wad structure of claim 5, wherein said skin portion substantially completely encapsulates said foam portion.
 7. The wad structure of claim 5, wherein said plastic foam portion has a density in the range of about 10 lb./ft3 to about 35 lb./ft.3
 8. A one piece plastic wad structure for use in a cartridge, said wad structure comprising: a tubular side wall of substantially solid plastic; and a transverse wall spanning said side wall to form a cup-shaped member, said transverse wall including a core of foam plastic unitary and in one piece with and of the same composition as said side wall, and a skin portion of substantially solid plastic unitary and in one piece with and of the same composition as said side wall and substantially completely encapsulating said foam portion of said transverse wall.
 9. A unitary wad structure for use in a cartridge, said wad structure comprising: an annular side wall of substantially solid plastic; and a transverse wall unitary and in one piece with said side wall and dividing the latter into back-to-back cups, said transverse wall including an inner core of foam plastic of the same composition as said side wall and an outer skin portion of substantially solid plastic of the same composition as said side wall, said skin portion substantially completely encapsulating said foam plastic core.
 10. A plastic wad structure for use in a cartridge, said wad structure comprising a one piece member having a projectile-receiving cup portion of substantially solid plastic; a gas obturating cup portion of substantially solid plastic; and a transverse wall unitary and in one piece with and separating said cup portions, said transverse wall being at least in paRt formed of foam plastic unitary and in one piece with and of the same composition as said solid plastic of said cups. 